1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and more particularly, to a method of forming an organic electroluminescent display device using sloped banks and a nozzle coating technique or ink-jet coating technique.
2. Discussion of the Related Art
An organic electroluminescent display device includes a cathode electrode injecting electrons, an anode electrode injecting holes, and an organic electroluminescent layer between the two electrodes. Namely, an organic electroluminescent diode has a multi-layer structure of organic thin films between an anode electrode and a cathode electrode. When forward current is applied to the organic electroluminescent diode, electron-hole pairs (often referred to as exciton) combine in an organic electroluminescent layer as a result of a P-N junction between the anode electrode, which provides holes, and the cathode electrode, which provides electrons. The electron-hole pairs have a lower energy together when combined than when they were separated. The energy gap between combined and separated electron-hole pairs is converted into light by an organic electroluminescent element. That is, the organic electroluminescent layer emits the energy generated due to the recombination of electrons and holes when a current flows.
As a result of the above principle, the organic electroluminescent device does not need an additional light source as compared to the liquid crystal display device. Also, the electroluminescent device is thin with light weight and high energy efficiency. The organic electroluminescent device has excellent advantages for displaying images, such as a low power consumption, a high brightness, a short response time and a light weight. Due to these advantages, the organic electroluminescent device is adapted to various electronic applications, e.g., mobile communication devices, PDA's (personal digital assistances), camcorders, and palm PC's. Because of the fabricating simplicity when forming the organic electroluminescent devices, the cost of production decreases as compared to the liquid crystal display device.
The driving method of operating the organic electroluminescent display device is classified as a passive matrix type and an active matrix type. The passive matrix type has a simple structure and fabrication process, but has a high power consumption compared to the active matrix type. Further, the passive matrix type is difficult to make large in size and has a decreasing aperture ratio as the bus lines increase.
In contrast, the active matrix type organic electroluminescent device provides a high display quality with high luminosity as compared with the passive matrix type. The core element of the organic electroluminescent display device is an organic electroluminescent (EL) material, such as a low or high molecular weight organic EL material. It is easy to handle the high molecular organic EL material rather than the low molecular weight organic EL material, and the high molecular weight organic EL material has good thermal resistance.
FIG. 1 is a schematic cross-sectional view illustrating an active matrix type organic electroluminescent display device according to the background art.
As shown in FIG. 1, an organic electroluminescent display device 10 includes first and second substrates 12 and 28 which are attached to each other by a sealant 26. On the first substrate 12, a plurality of thin film transistors (TFTs) T and array portions 14 are formed. A first electrode (i.e., an anode electrode) 16, an organic luminous layer 18 and a second electrode (i.e., a cathode electrode) 20 are sequentially formed on and over the TFTs T and the array portion 14. At this point, the organic luminous layer 18 includes red (R), green (G) or blue (B) color in each pixel P and thus each pixel P emits light of red (R), green (G) or blue (B) color. Namely, to show color images, organic color luminous patterns are disposed respectively in each pixel P. Further, the organic luminous layer 18 is formed by patterning or printing color organic material in each pixel P.
Still referring to FIG. 1, the second substrate 28, which is attached to the first substrate 12 by the sealant 26, includes a moisture absorbent 22 on the rear surface thereof. The moisture absorbent 22 absorbs the moisture that may exist in the cell gap between the first and second substrates 12 and 28. When disposing the moisture absorbent 22 in the second substrate 28, a portion of the second substrate 28 is etched to form a recess. Thereafter, the powder-type moisture absorbent 22 is disposed into this recess and then a sealing tape 25 is put on the second substrate 28 to fix the powder-type moisture absorbent 22 into the recess.
In the above-mentioned structure and configuration, a nozzle coating technique, for example, is utilized to form the organic luminous layer. In the nozzle coating technique, separators or banks are used to form the separate red (R), green (G) and blue (B) organic luminous layer. If the major axis of the pixel is defined as a longitudinal direction and the minor axis of the pixel is defined as a transverse direction, the nozzle moves in rectilinear and reciprocating motion along the longitudinal direction to form the color organic EL material, and thus, the organic luminous layer has a linear shape with a desired width. As a result, the pixels that are arranged in the longitudinal direction have the same color EL material together and emit the same light color. Furthermore, the pixels that are arranged in the transverse direction have the red (R), green (G) and blue (B) colors alternately. At this time, longitudinal banks are disposed in the longitudinal direction between the red (R), green (G) and blue (B) organic luminous layers in order to prevent interferences between the red (R) and green (G), green (G) and blue (B), or blue (B) and red (R) EL materials.
The formation of the longitudinal banks is conducted through the photolithography process or the printing process, for example. When the photolithography method is used to form the longitudinal banks, the organic material is first formed over the substrate by spin coating or spray coating and then a photoresist is deposited on the organic material. Thereafter, a mask having the shape corresponding to the longitudinal banks is disposed over the photoresist for the light exposure. After the light exposure through the mask, the exposed photoresist is developed to have the bank shape. Then the organic material is etched to be the longitudinal banks.
FIG. 2 is a schematic plan view illustrating an organic electroluminescent device having longitudinal banks according to the background art. As shown, a plurality of longitudinal banks 50 are disposed in a longitudinal direction. A plurality of pixels 60 are disposed in between the longitudinal banks 50. In each pixel 60, a thin film transistor T and a pixel electrode 57 are located. When forming a color organic luminous layer 62 in between two longitudinal banks 50, a nozzle injecting the organic EL material moves in a longitudinal direction and then the organic EL material is coated over the substrate. At this time, it is quite difficult to make the color organic luminous layer 62 have a uniform thickness and a thickness stability because the nozzle's scan speed and the injecting quantity of organic EL material are very different and variable during the nozzle movement.
FIG. 3 is a schematic perspective view illustrating a nozzle injection for forming a luminous polymer over a substrate between longitudinal banks according to the background art. The plurality of pixels 60 are formed over the substrate 100, and the plurality of longitudinal banks 50 are disposed over the substrate in a longitudinal direction while dividing the longitudinally arranged pixels 60. A nozzle 80 injects a luminous polymer 90 between the longitudinal banks 50. When leveling the luminous polymer 90 after the nozzle injection, the injected luminous polymer 90 streams down from the edge of the substrate 100 that is at the end of a trench between two longitudinal banks 50. Therefore, the thickness of the injected luminous polymer 90 becomes smaller, and leveling the luminous layer 90 is very difficult. Furthermore, the thickness of the luminous layer 90 is not uniform and varies between the longitudinal banks 50 over the substrate 100.
To overcome this problem, the scan speed of the nozzle 80 is sometimes reduced. However, the decrease of scan speed causes overflowing over the longitudinal banks so that the luminous polymer 90 affects the luminous polymer of adjacent pixels. If the scan speed of the nozzle 80 increases, the luminous layer 90 is formed within the trench between two longitudinal banks 50, but may be very thin in thickness. Thus, the completed organic electroluminescent device can have poor operating characteristics.
Furthermore, the thickness and leveling of the luminous layer 90 is affected by the shape of longitudinal banks 50. FIG. 4 is a cross-sectional view taken along a line IV-IV of FIG. 3. As shown in FIG. 4, the longitudinal banks 50 have a rectangular cross-section. Thus, the luminous polymer 90 rises high against the longitudinal bank 50 in a contacting portion S between the luminous polymer 90 and the longitudinal bank 50 due to the surface tension of the liquid phase luminous polymer 90. As a result, the luminous polymer 90 has a thick thickness in the area contacting the longitudinal bank 50, and a poor leveling condition exists. Consequently, the thickness non-uniformity and poor leveling of the luminous layer 90 makes the organic electroluminescent device have a poor light-emitting degree and efficiency. Further, the life span of the luminous layer 90 is shortened.